A general receiver adopting a super-heterodyne method converts a frequency by using a mixing circuit after amplifying a modulated wave signal received via an antenna at a high frequency, and demodulates it after converting it into an intermediate-frequency signal having a predetermined frequency. For instance, an FM receiver in Japan is using a local oscillation signal deviated from a carrier frequency of the modulated wave signal by 10.7 MHz, where FM detection and stereo demodulation processes are performed to the intermediate-frequency signal of 10.7 MHz.
In particular, in some data communication fields, a technique called a direct conversion method using the local oscillation signal almost equal to the modulated wave signal, and such a receiver demodulates the intermediate-frequency signal of a low frequency.
Incidentally, the above-mentioned receiver in the past has a problem that, if a signal leaked via printed wiring of a local oscillator for generating the local oscillation signal sneaks on an input side of the modulated wave signal, it becomes noise and appears in the intermediate-frequency signal. In particular, in the case of the receiver adopting the direct conversion method, the frequencies of the modulated wave signal and local oscillation signal are close, and so the same frequency component as the local oscillation signal is superimposed as noise on the modulated wave signal. Furthermore, in the case where this noise component is included in a frequency bandwidth of the modulated wave signal, the noise component cannot be eliminated from the intermediate-frequency signal so that quality of a receiver signal deteriorates.